Grain size statistics, texture, and grain boundary distribution are microstructural characteristics that influence materials properties. These properties include strength, resistivity, and magnetic susceptibility. The microstructural characteristics are usually derived from an orientation map which displays crystallographic orientations of grains in the microstructure. Orientation maps are obtained by performing orientation imaging microscopy (OIM) using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). Orientation imaging microscopy (OIM) using a scanning electron microscope (SEM) is generally performed for materials with grain sizes greater than 60 nm. However, the accuracy of the orientation maps reduces when the grain sizes are less than 60 nm, which is the case with nanomaterials. In these situations, TEMs are used to obtain orientation maps. The OIM techniques are generally performed using a TEM for nanomaterials. These techniques use calculations from the kinematical diffraction theory due to its simplicity. In practice, the electron diffraction in TEM does not necessarily follow a behavior predicted by the kinemetical theory.
In literature, four methods have been proposed for performing orientation imaging microscopy (OIM) using TEMs: (1) conical scan method, (2) microbeam spot pattern method, (3) Kikuchi method, and (4) precession method. The first three methods provide correct orientation maps in some cases, however, they have limited applicability for a routine use. Others have applied the conical scan method to a platinum thin film sample but failed to obtain correct orientation maps. (Darbal, A., Barmak, K., Nuhfer, T., Dingley, D. J., Meaden, G., Michael, J., Sun, T., Yao, B., & Coffey, K. R. (2009). Orientation imaging of nanocrystalling platinum films in the TEM. Microscopy and Microanalysis, 15, (2), 1232-1233). Others have discussed drawbacks of the microbeam spot pattern method. (Zaefferer, S. & Wu, G. (2008). Development of a TEM based orientation microscopy system. In Application of Texture Analysis, Proceedings of International Conference on Texture of Materials—15, pp. 221-228. New Jersey: Wiley-American Ceramic Society). These have also argued that the Kikuchi method has limited applicability to nanometerials (Wu, G. & Zaefferer, S. (2009) Ultramicroscopy, 109, (11), 1317-1325). Additionally, the Kikuchi method does not accurately determine grain direction in thinner materials because of limitations to the method. Advances in TEM orientation microscopy by combination of dark-field conical scanning and improved image matching. They also attributed the limited applicability to weak intensities of Kikuchi lines due to lattice defects and small grain sizes.
The latest method, known as the precession method, generates spot diffraction patterns in a modified TEM setup requiring additional hardware. The TEM must be modified with the additional proprietary hardware, which is expensive and time consuming to change the TEM back to its unaltered state for other TEM imaging. Subsequently, the precession method applies calculations from the kinemetical diffraction theory to produce orientation maps. This is a relatively new method and a critical evaluation of this method on different material samples is due. However, there are potential problems with this method. First, the physical modifications with additional hardware to the TEM add to the cost of the setup and are often difficult to procure. In addition, authors treated the precession diffraction patterns as the kinemetical diffraction patterns. This treatment may not be correct for certain cases.